1. Field of the Invention
The present invention relates to a vocoder system and, in particular, to a vocoder system and method for vocal sound synthesis, with which it is possible to improve the performance expression of a sound with a light computational load.
2. Description of the Prior Art
Vocoder systems have been known with which the formant characteristics of a speech signal that is input are detected and employed. Using a musical tone signal produced by operating a keyboard or the like, the musical tone signal is modulated by the speech signal, outputting a distinctive musical tone. With this vocoder system, the speech signal that is input is divided into a plurality of frequency bands by the analysis filter banks, and the levels of each of the frequencies that express the formant characteristics of the speech signal that are output from the analysis filter banks are detected. On the other hand, the musical tone signal that is produced by the keyboard and the like is divided into a plurality of frequency bands by the synthesis filter banks. Then, by amplitude modulation with the envelope curves that correspond to the output of the analysis filter banks, an effect such as that discussed above is applied to the output sound.
However, with the vocoder systems of the past, since the characteristics of each of the filters (the center frequency and bandwidth) of the analysis filter bank and the synthesis filter bank have been set to be equal, the formant characteristics of the speech signal are reflected as they are, unchanged, in the output sound. Thus, it has not been possible to change the formant of the speech that has been input and modulate the output of the synthesis filters. In other words, with the vocoder systems of the past, there is the problem that it is not possible to apply sound changes to the output sound using the sex, age, singing method, special effects, pitch information, strength, and the like. The performance expression of the output sound is, therefore, limited.
To solve this problem, there is a method in which the center frequencies of each of the filters that comprise the synthesis filter bank are changed with respect to the center frequencies of each of the filters that comprise the analysis filter bank. By means of this method, the formant characteristics of the speech signal can be shifted on the frequency axis and changed. It is thus possible to improve the performance expression of the output sound. It is set up, for example, with the speech signal divided into a plurality of frequency bands by the analysis filter bank and, in a specified time t, as is shown in FIG. 7(a), a formant curve in which the low range side is rich is detected. In this case, when the center frequencies of each of the filters that comprise the synthesis filter bank are changed so as to become a specified percentage higher than the center frequencies of each of the corresponding filters that comprise the analysis filter bank, the formant characteristics of the output sound that corresponds to FIG. 7(a) are changed, as is shown in FIG. 7(b), so as to be drawn toward the high frequency side on the frequency axis. Therefore, the formant characteristics of the male voices, which are rich on the low range side, can be shifted to the high range side and changed to the formants of female or children's voices.
On the other hand, in those cases where, contrary to what has been discussed above, the formant curve that is produced from the output from the analysis filter bank is, as is shown in FIG. 9(a), rich on the high range side, when the center frequencies of each of the filters on the synthesis side are changed so as to become a specified percentage lower than the center frequencies of each of the corresponding filters on the analysis side, the formant characteristics of the output sound that corresponds to FIG. 9(a) are changed, as is shown in FIG. 9(b), so as to be drawn toward the low frequency side on the frequency axis. Therefore, the formants of female voices, which have formant characteristics that are rich on the high range side, can be shifted to the low range side and changed to the formants of male voices.
If the center frequencies of each of the filters that comprise the synthesis filter bank are changed in this manner with respect to the center frequencies of each of the corresponding filters that comprise the analysis filter bank, it is possible for the formant characteristics of the speech signal to be changed and for this to be reflected in the output signal, and the performance expression of the output signal can be improved. In Japanese Unexamined Patent Application Publication (Kokai) Number 2001-154674, a vocoder system is disclosed that is related to this method in which the frequency band characteristics (the center frequencies) of the synthesis filter bank are changed appropriately and that has been furnished with a parameter setting means in which parameters are set in order to determine the frequency band characteristics of the synthesis filter bank.
However, in those cases where the method discussed above is employed in order to improve the performance expression of the output sound, the filter coefficients of each of the filters that comprise the synthesis filter bank must be changed. When this is carried out with digital filters, the computational load that is borne by the processing unit for the computation becomes great. In addition, since the synthesis filter bank is actually on the side on which the output sound is produced, in order to prevent the generation of noise, it is necessary to change the filter coefficients for each sample and do the computation; thus, the computational load on the processing unit becomes even greater.
In addition, in those cases where the method discussed above is employed when the formant characteristics are changed during the performance, it is necessary to change the filter coefficients of each of the filters that comprise the synthesis filter bank individually and continuously. Therefore, the computations of the processing unit become complicated and the computational load becomes great.
The present invention resolves these problems and has as its object a vocoder system with which it is possible to improve the performance expression of the output sound with a light computational load.